Steam will have automatically sent you an email with this validation code, which you then need to input at the command prompt.
(This will download and install the required steam files to your custom steam directory)
The following instructions will guide you through setting up one Arma 3 server on 1 box, however they will also set the foundations for installing multiple servers on that same box: If you are in doubt, contact your Internet Service Provider. Arma 3 server does not support IPv6 or DSlite IPv4 via IPv6 tunnel.